1. Field of the Invention
The present invention relates to a hydraulic power steering apparatus whose source of steering assisting force is hydraulic actuator, and more particularly to a power steering apparatus having a hydraulic pressure reaction apparatus which restricts control operation of a hydraulic control valve for controlling hydraulic pressure being supplied to aforesaid hydraulic actuator.
2. Description of Related Art
A hydraulic power steering apparatus, whose source of steering assisting force is a hydraulic actuator such as a hydraulic cylinder being provided in a steering mechanism, has such construction as follows. In the case where an input shaft being connected to a steering wheel and an output shaft connecting operationally to a steering front wheel are connected on a coaxial line through a torsion bar and then steering torque is exerted upon the steering wheel, relative angular displacement is adapted to be created between the two shafts according to torsion of the torsion bar accompanying the above case. In addition, there is provided at aforesaid connected portion with a hydraulic control valve comprising a cylindrical casing which rotates interlockingly with one of the shafts and a valve body which is fitted into the casing coaxially therewith and which rotates interlockingly with the other shaft. The hydraulic pressure being supplied to aforesaid hydraulic actuator is controlled by operation of the hydraulic control valve according to the direction and magnitude of aforesaid steering torque, thereby steering assisting force being obtained.
It is well-known that the magnitude of force being required for the steering operation for steering corresponds to the magnitude of reaction force acting upon the steering front wheel from the surface of a road, and that this magnitude of reaction force corresponds to vehicle speed. On the other hand, in aforesaid power steering apparatus, corresponding relationship between steering torque being exerted upon the steering wheel and the steering assisting force being generated by the hydraulic actuator is decided only by the torsion characteristics of said torsion bar.
Accordingly, in the case where this torsion bar is selected on the basis of big reaction force of the vehicle which is stopped or runs at a low speed running, there is a problem that its steering is carried out by a slight power being exerted upon the steering wheel at its high speed running, resulting in aggravation of rectilinear propagation stability. On the contrary, in the case where the torsion bar is selected on the basis of its high speed running, there is a problem that sufficient steering assisting force cannot be obtained at the stopped state and at the low speed running of the vehicle.
In order to solve above-mentioned problems, there has been developed a power steering apparatus being provided with a hydraulic pressure reacting portion at the connected portion of said input shaft with the output shaft, the hydraulic control valve being provided in parallel relation therewith. According to the hydraulic reacting portion, hydraulic pressure reaction whose magnitude is variable corresponding to the vehicle speed is applied between the both shafts, thereby control operation of said hydraulic control valve being created according to the relative angular displacement of the both shafts, is restricted by strong power at its high speed running, so is by weak power at its low speed running.
FIG. 1 is an enlarged cross sectional view of major parts showing an example of the conventional hydraulic pressure reacting portion. The hydraulic pressure reacting portion is provided in a cylindrical housing 10 supporting both of the input and output shafts at their connected portion. In the figure, reference numeral 30 designates a cylindrical portion being disposed so as to be connected coaxially with the end of the connected portion of the output shaft for example, the cylindrical portion being fitted rotatably into the housing 10. Between the cylindrical portion 30 and the inner peripheral surface of the housing 10, there is formed a reaction chamber 33 as a ring space having an appropriate width in the axial direction. Inside of the cylindrical portion 30, a part of an input shaft 2 is loosely fitted, and on the outer peripheral surface of the part, a plurality of concave portions 22, 22 . . . are formed in a circumferential direction at equal interverals with each other. In addition, in cylindrical portion 30, through holes 31, 31 . . . which pass through in the radial direction are formed at the circumferential direction at almost equal intervals with each other at the positions corresponding to the respective concave portions 22, 22 . . . . Respective plungers 32, 32 . . . are fitted in these respective through holes 31, 31 . . . so as to be slidable along them. Respective plungers 32, 32 . . . are provided with respective semi-spherical convex portions at the inner end thereof, the convex portions being engaged with the concave portions 22, 22 . . . being formed at the outer circumference of the input shaft 2 as shown in the figure.
In the figure, reference numeral 4 designates a torsion bar connecting the input shaft 2 to the output shaft. When steering torque is exerted upon the steering wheel, relative angular displacement accompanying the torsion of the torsion bar 4 is adapted to be created between the input shaft 2 being connected to the steering wheel and the output shaft whose rotation is restricted by the reaction force being acted upon the steering front wheel from the surface of a road. As described earlier, by the operation of the hydraulic control valve (not shown) which is arranged at the connecting portion of both shafts so as to be in parallel relation with the hydraulic reacting portion, the hydraulic pressure being supplied to the hydraulic actuator in use for steering assisting is controlled according to the relative angular displacement.
On the other hand, plungers 32, 32 . . . at the hydraulic reacting portion is pressed against to the inwardly radial direction thereof by hydraulic pressure inside of the reaction chamber 33, acting upon the outside end surfaces of plungers 32, 32 . . . , thereby the inside end portions thereof being engaged with the concave portions 22, 22 . . . on the outside circumference of the input shaft 2. Consequently, the relative angular displacement between the input shaft 2 and the cylindrical portion 30 forming a part of the output shaft is restricted by force corresponding to the hydraulic pressure. Accordingly, it seems as if reaction force corresponding to the hydraulic pressure is acting upon the output shaft side. Until the steering torque being exerted upon the input shaft 2 through the steering wheel reaches a magnitude which forces up the plungers 32, 32 . . . in an outwardly radial direction against the hydraulic pressure, the relative angular displacement is not created and operation of the hydraulic control valve is not carried out, as the result, the hydraulic actuator does not generate the steering assisting force. Accordingly, by introducing a hydraulic pressure which becomes high and low correspondingly to the vehicle speed into the reaction chamber 33, an appropriate rigidity is applied to the steering wheel at a high speed running of a vehicle, resulting in realizing the rectilinear stability to be improved. At the same time, it is also realized that the sufficient steering assisting force can be obtained at the stopped state and at the low speed running of the vehicle so as to reduce the power being required for steering operation as much as possible.
For means to make introduced hydraulic pressure to the reaction chamber 33 correspond to the vehicle speed, various kinds of proposals have been made. In a power steering apparatus disclosed in U.S. Pat. No. 4,796,715, an oil path being connected to a pressure oil tank through a variable throttle valve and a fixed throttle valve is provided at a discharge side of a hydraulic pump for generating a predetermined hydraulic pressure. The hydraulic pressure between the both throttle valves in the middle of the oil path is utilized as the introduced hydraulic pressure to the reaction chamber 33, and degree of throttle opening of the variable throttle valve is made to be variable according to the detected result of the vehicle speed sensor, whereby the introduced hydraulic pressure corresponding to the vehicle speed is obtained. In Japanese Patent Publication No. 53-45571 and in Japanese Utility Model Publication No. 60-38219 by an applicant of the invention, it is disclosed that a generated hydraulic pressure of a hydraulic pump (gear pump is used in the former publication and trochoid pump in the latter publication) being installed at an output shaft of a transmission and being driven by a speed corresponding to the vehicle speed, is directly introduced into the reaction chamber as hydraulic pressure corresponding to the vehicle speed.
FIG. 2 is a graph showing the correlation between steering torque being exerted upon the steering wheel of the power steering apparatus being provided with such a hydraulic pressure reacting portion as above constructed and the steering assisting force being generated due to the steering torque by the hydraulic actuator for steering assisting. As shown in this figure, in the conventional power steering apparatus, by the operation of the hydraulic pressure reacting portion, steering torque is kept to be almost zero until the steering assisting force reaches a predetermined magnitude, and after that, rapidly increasing steering torque is obtained. In addition, a characteristic is obtained that a raised point of the steering assisting force shifts to the side where steering torque is larger as the vehicle speed becomes high because the introduced hydraulic pressure to the reaction chamber 33 is made to correspond to the vehicle speed.
Now in the case where the actual steering feeling is considered, it is desirable for the rigidity of the steering wheel to increase as the steering angle increases. However, the conventional power steering apparatus, as apparent from the FIG. 2, has a characteristic that the steering assisting force increases rapidly. That is, after the steering torque acting upon the steering wheel passes the raised point, the rigidity of the steering wheel is kept almost constant, resulting in a problem to create slight incompatibility of steering feeling. In order to solve this problem, a characteristic that the steering torque shows a relatively slow increase manner is required after the steering torque passes the raised point. Such a characteristic is possible to be realized by changing the opening degree of the variable throttle valve according to the detected result of the steering angle as well as the detected result of the vehicle speed in a power steering apparatus disclosed in the U.S. Pat. No. 4,796,715. However, the construction of the control unit for performing change of opening degree becomes complicated as well as a sensor for detecting steering angle is required besides the vehicle sensor, resulting in a problem that malfunction is possibly generated at each part. In comparison with the above apparatus, in a power steering apparatus which utilizes a generated hydraulic pressure of a hydraulic pump (hereinafter referred to as a vehicle speed sensor pump) for generating a hydraulic pressure corresponding to the vehicle speed, there is no problem that the malfunction is created, however, it has been considered to be difficult to realize aforesaid desirable characteristic.